Astronomers using NASA's Hubble Space Telescope have discovered a ghostly ring of dark matter that formed long ago during a titanic collision between two massive galaxy clusters. The ring's discovery is among the strongest evidence yet that dark matter exists. Astronomers have long suspected the existence of the invisible substance as the source of additional gravity that holds together galaxy clusters. Such clusters would fly apart if they relied only on the gravity from their visible stars. Although astronomers don't know what dark matter is made of, they hypothesize that it is a type of elementary particle that pervades the universe.
This Hubble composite image shows the ring of dark matter in the galaxy cluster Cl 0024 17. The ring-like structure is evident in the blue map of the cluster's dark matter distribution. The map was derived from Hubble observations of how the gravity of the cluster Cl 0024 17 distorts the light of more distant galaxies, an optical illusion called gravitational lensing. Although astronomers cannot see dark matter, they can infer its existence by mapping the distorted shapes of the background galaxies. The map is superimposed on a Hubble Advanced Camera for Surveys image of the cluster taken in November 2004. PHOTO CREDIT: NASA

If it weren't for the powerful gravity in some ghostly unknown stuff that surrounds us, everything and everybody in our universe would fly apart in a flash.

The mysterious stuff is called "dark matter," and the grip of its gravity holds all the stars and planets, people and atoms exactly where they belong, just the way the laws of physics dictate.

Now astronomers using the Hubble Space Telescope reported Tuesday that they have discovered convincing new evidence for the existence of dark matter -- in a huge ring circling the remains of two clusters of distant galaxies that clashed in a monstrous collision nearly 2 billion years ago.

The remains of those galaxies are about 5 billion light-years away, but the gravity in the dark matter around them has so distorted the telescope's view of objects beyond them that the astronomers say their complex calculations enabled them to determine the shape of the ring and its mysterious contours. "We think this is the strongest evidence yet for the existence of dark matter," said astrophysicist M. James Jee of Johns Hopkins University in a phone conference with reporters from the Space Telescope Science Institute, the Hubble's ground headquarters near Baltimore.

"This is the first time we have detected dark matter as having a unique structure that is different both from the gas and the galaxies in the cluster," he said.

His team's formal report will be published June 1 in the Astrophysical Journal.

Stanford astrophysicist Blas Cabrera, who has been hunting for fleeting particles of dark matter with a specialized detector located a half-mile down an old Minnesota mine, agreed after reading the Hubble report.

"Thus," Cabrera told The Chronicle, "we have more and more convincing evidence that the dark matter is real material -- probably elementary particles. Now we need to detect those particles directly with laboratory experiments."

Richard Massey, a Caltech astrophysicist who was also not part of the Hubble team but who joined Jee in the teleconference, was just a tad dubious.

He said he was hugely impressed by Hubble's ability to capture images of the distant galaxy cluster and said the telescope's observations are "really valuable." But the report of the dark-matter ring "is meeting some skepticism in the astronomical community," he said. "It might be a spurious artifact, so I want to see more verification. But it's really exciting if it's right."

In August, astronomers using NASA's orbiting Chandra X-ray Observatory reported that they had detected dark matter in the remains of a different long-ago collision of two giant galaxy clusters 3 billion light-years away, and known as the Bullet Cluster. But that distant cluster gave Chandra a view from a tilted angle, and was not as clear as the Hubble's head-on observation of its target, which is known only by its catalog designation of Cl 0024+17.

There have been other brief hints by astrophysicists that dark matter is the real McCoy, but no certainty, and scientists who contend it must be made of unknown elementary particles have given those particles fanciful names based on their presumed properties -- among them neutralinos, axions, MACHOS for Massive Compact Halo Objects and WIMPS, for Weakly Interacting Massive Particles.

So the concept of dark matter is weird indeed because it can't be seen, reflects no light, and its shape can only be inferred by observation of its effects -- as in its effect on the shape of the most recently observed galaxy clusters.

In cosmology and astrophysics, ordinary matter -- everything we are, see and feel -- from grit to galaxies -- comprises only about 4 percent of the universe. Dark matter, whose gravity holds all ordinary matter together, makes up another 21 percent of the universe.

But since the universe is known to be expanding faster and faster, even while its dark matter holds it together, there must be some repulsive force to keep the expansion speeding. Physicists call that force dark energy, and their equations tell them that dark energy must be the most dominant feature of all -- comprising a good 75 percent of everything in the universe.

In past decades, increasingly sophisticated telescopes and astrophysical theories have determined that there must be a lot of "missing mass" in the universe holding all the galaxies, stars and ordinary matter together, because calculations of the total gravity in the observable universe did not show there was enough.

Edwin Hubble discovered the expansion of the universe in the 1920s, and over time scientists have become aware that the universe is even expanding more and more rapidly. So the puzzle has been to explain what pushes the stars and galaxies apart so fast while some mysterious gravitational force keeps them from flying away altogether.

In the past 20 years -- both from observations of the remnants of the Big Bang some 13.7 billion years ago and the calculations of cosmologists -- scientists believe they have found the answer: Dark matter holds things together, and dark energy speeds the expansion.

The Hubble team made its observations of dark matter last year, using the telescope's Advanced Camera for Surveys. But that camera has since malfunctioned. It will be able to continue the dark-matter quest across far-off skies only if a space shuttle servicing mission returns to the Hubble next year and makes repairs.